Amplifiers include automatic gain control (AGC) circuits to ensure that the power level output signal remains constant. Typically, a quadrature amplitude modulation (QAM) modulated signal is transmitted from a headend facility and transmitted downstream to amplifiers for amplification and subsequently to subscriber equipment for final processing. An amplifier having a single pilot QAM AGC detector typically includes a bandpass filter (BPF) that has a 3 dB (decibel) bandwidth narrower than the QAM signal bandwidth, which is transmitted in a channel, to ensure that the BPF has a sufficient out-of-band rejection to reject the signal in the adjacent channels.
At times, however, the QAM modulation is turned off resulting in a continuous wave (CW) signal, which has, in concept, a 3 dB bandwidth of 0 Hz (Hertz). So when the QAM modulation is turned off, the signal power level filtered by the BPF and sensed by the AGC circuit is somewhat higher than when the QAM modulation is on. The exact amount depends on the system requirement and the design of the filter, but it is typically between 1 and 6 dB. Consequently, since the AGC circuit determines the output power level within the entire channel, the amplifier then incorrectly adjusts the power level lower by the same amount, e.g., 1-6 dB. Therefore, what is needed is a system and method for detecting the on and off condition of the QAM modulation as well as maintain the desired output power level of the amplifier regardless whether the QAM modulation is on or off.